The present invention relates to near-field scanning optical microscopes using high Q-factor piezoelectric sensing elements.
Near-field scanning optical microscopy is capable of producing optical images with resolutions surpassing the half-wavelength limit of conventional far-field microscopy. The probe of a near-field scanning optical microscope (NSOM) is a small aperture, usually the tip of a sharpened optical fiber, scanned in close proximity to the sample""s surface. The aperture serves as either a light-source illuminating the sample, or as an aperture collecting light emitted or scattered from the sample. In order to achieve subwavelength resolution, the aperture-sample distance must be scrupulously controlled by an appropriate feed-back strategy. Height control is most frequently accomplished optically; that is, the fiber tip is caused to vibrate at one of its resonant frequencies. A laser beam is focussed onto the fiber, and the height-dependent transverse vibrational amplitude of the tip is measured using, for example, a photodiode. 1 The amplitude is then used as the feedback parameter used to maintain the tip at approximately a fixed distance above the surface. An alternative control scheme, introduced recently by Karrai and Grober2, utilizes a quartz micro-tuning fork as the height-sensing element. As the tip of the optical fiber (which is attached to the tuning fork) approaches the sample surface, the amplitude, phase and frequency of the tuning fork""s vibrational normal modes change. Any one of these parameters can then be used as the feedback signal for tip-sample distance control. In the design of ref. 2 the tuning fork was used strictly as a sensing element; an additional piezo element was used to dither the optical fiber/tuning fork assembly parallel to the sample surface. Another strategy, introduced by Atia and Davis3, makes use of the tuning fork as both the dithering and the sensing element. Piezoelectric tip-sample distance regulation yields a number of advantages over optical control including simplicity, compactness, less drift in the feedback loop and lower levels of parasitic optical background.
Since the original publication, a number of NSOMs utilizing piezoelectric height control have been described.4,5,6,7,8,9,10 Most operate in the so-called xe2x80x9cshear-force modexe2x80x9d. In this configuration, an optical fiber is attached along the edge of one of the tuning fork""s tines, and the tuning fork is oriented such that the tip vibrates parallel to the sample surface. In an alternative orientation the fiber tip vibrates approximately along the normal to the sample surface but without making contact with the sample surface. (We will refer to this mode of tip motion as a perpendicular mode of operation.) In this mode of operation the tip senses higher force gradients resulting in more reliable tip-sample distance control. To the best of our knowledge, only two NSOM instruments with piezoelectric sensing elements in which the tip vibrates along the normal to the surface have been described so far. Muramatsu et al.7 describes an instrument in which the perpendicular mode of operation is achieved by bending the tip of the optical fiber glued along the edge of a tine by approximately 90xc2x0. Tsai and Lu10 attached an optical fiber across a tine and used a piezoelectric bimorph to dither the tuning fork/fiber assembly normal to the sample surface. However, the quality factors of the tuning fork/optical fiber assemblies described by both groups were substantially lower than the Q-factors normally reported for shear force instruments. Hence, the perpendicular mode of operation in both of these instruments was achieved at a cost in the overall instrumental performance from those achieved in the shear mode configuration. In addition, bending the optical fiber in [7] potentially results in optical losses.
The magnitude of the Q-factor of the tuning fork/optical fiber assembly is a central factor influencing the performance of a near-field microscope with piezoelectric sensing elements. Commercially available quartz micro tuning forks have very high Q-factors, ranging from 50,000 to 200,000 in vacuum and 10,000 to 20,000 in air. Attaching an optical fiber to one of the tines of a tuning fork causes a large reduction in Q-factor. For NSOMs operating in the shear force mode, Q-factors of 1,000 to 3,000 have been reported. By contrast the Q-factors of instruments operating in the perpendicular mode described in the literature so far did not exceed several hundred. The minimum force (or force gradient) detectable by an instrument is proportional to Qxe2x88x92xc2xd; hence a large Q is crucial if high sensitivity increase is to be achieved.11 Hence, in the instruments described so far, the potential gain in instrument sensitivity normally achieved by operating the NSOM in the perpendicular mode was significantly offset by their lower Q-factors. Moreover, because stable self-sustained oscillations are difficult to achieve with low-Q-factor piezoelements, additional dithering elements had to be used to cause the tuning fork/optical fiber system to vibrate thereby complicating the device and introducing an additional source of drift in the feedback signal.
It is an object of the present invention to provide a optical fiber/tuning fork sensor assembly for use in a near field optical microscope having a Q-factor superior to currently available sensor assemblies
In one aspect of the invention there is provided a piezoelectric sensing element for use in a near-field scanning optical microscope, comprising:
a micro tuning fork mounted in a holder, the micro tuning fork including first and second tines and the holder having a piezoelectric element for dithering the micro tuning fork; and
an optical fiber being connectable to a photodetection means and having a thinned end portion having a diameter less than a threshold thickness, the thinned portion of the optical fiber being attached to said first tine with said first tine being adapted to be adjacent to a surface being scanned in operation.
In another aspect of the invention there is provided a piezoelectric sensing element for use in a near-field scanning optical microscope, comprising:
a micro tuning fork mounted in a holder, the micro tuning fork including first and second tines and the holder having a piezoelectric element for dithering the micro tuning fork; and
an optical fiber being connectable to a photodetection means and a thinned end portion having a diameter less than a threshold thickness, the thinned portion of the optical fiber being attached at a first position thereof to the holder and being attached at a second position thereof spaced from an end of the optical fiber to a first tine at a position spaced from an end of the first tine, the optical fiber extending transversly across the tines of the micro tuning fork, said first tine being adapted to be adjacent to a surface being scanned in operation.
In another aspect of the invention there is provided a A piezoelectric sensing element for use in a near-field scanning optical microscope, comprising:
a micro tuning fork mounted in a holder for operation in a shear mode, the micro tuning fork including a pair of tines and the holder having a piezoelectric element attachable thereto for dithering the micro tuning fork; and
an optical fiber having an end portion connectable to a photodetection means and a thinned end portion having a diameter in a range of about 40-80 xcexcm, the thinned portion of the optical fiber being attached along a length of one of said tines.